1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to a recordable optical disk of a single plate type comprising a recording layer containing a dye and a metal reflective layer overlying the recording layer which is recordable and has playability (i.e. capability of being reproduced) by commercially available compact disk players.
2. Description of the Related Art
An optical recording medium capable of writing comprising a recording layer containing a dye such as phthalocyanine dye and the like and a metal reflective layer overlying the recording layer for enhancing reflectivity is disclosed, for example, in U.S. Pat. No. 4,298,975. However, said U.S. patent mentions nothing about playability by commercially available compact disk (CD) players and does not disclose conditions under which there is produced a medium excellent in jitter value and error rate when a pit length recording is conducted.
Optical recording mediums of a single plate type having a high reflectivity and playability by commercially available CD player have been recently proposed, for example, in Optical Data Storage 1989 Technical Digest Series, Vol. 145 (1989), EP 353393 and the like. In particular, there are disclosed dyes used as a recording layer and characteristics of the reflective layer for attaining a high reflectivity.
U.S. Pat. No. 4,990,388 discloses a recording mechanism for a recording layer of an optical recording medium having playability by CD players.
In such optical recording mediums, it is necessary for the mediums to give tracking error signals for track control and radial contrast signals for access to an aimed track.
When a recording layer is formed by a vacuum technique such as sputtering and the like, the recording layer is formed strictly following the shape of groove of the substrate, and therefore, optimization of the above-mentioned tracking error signals and radial contrast signals can be effected by simulation.
However, in the case of an optical recording medium having a layer formed by coating a dye as a recording layer, the surface shape of the recording layer does not follow the groove shape of the substrate, and therefore, it is difficult to design the optimum shape by simulation as mentioned above, and the above-mentioned prior patents disclose nothing about it.
The relation between a tracking error signal and a groove shape of a substrate is disclosed, for example, in Japanese Patent Application Laid-open No. Hei 3-22224. However, the disclosure is not sufficient to attain a complete playability by commercially available CD players.
Among commercially available CD players, a player carrying a 3-beam head has no problem, but the present inventors have found that playability by a player carrying a one beam head is problematic though the reason is not clear.
In addition, with respect to radial contrast signals, the optimization therefore has not yet been reached, and radial contrast signals are not used for access. Instead, an external sensor or the like is used though the access speed is slow.
According to the present inventors' investigation, with respect to playability by CD players and radial contrast signals, it is not enough to optimize the shape of the groove.
In the case of an optical recording medium capable of recording comprising a recording layer containing a dye formed by a coating method and a metal reflective layer overlying the recording layer, the reflection at the interface of the recording layer and the reflective layer is predominant. Therefore the shape of the surface of the recording layer and the thickness of the recording layer on the groove and that on the land become important, However, the shape of the surface of the recording layer and the thickness of the recording layer on the groove and that on the land vary depending on coating conditions such as concentration, surface tension and viscosity of the coating solution, boiling point and evaporation speed of the coating solvent, the manner of rotation in the case of spin coating, and the like, even if the shape of groove of the substrate is the same. Therefore, strict optimization has not yet been effected.
EP 0410183 discloses difference in light path length between the recording layer on the groove portion and that on the land portion and degree of modulation. However, it mentions nothing about the problem of playability by commercially available CD players and radial contrast singnals.
On the contrary, there are generally two methods for recording information in an optical recording medium, that is, pit position recording and pit length recording. In the case of a CD, pit length recording is effected with pits of 9 lengths. In pit length recording, for the purpose of lessening error rate, it is very important to effect recording pits of various lengths with low jitter value (low fluctuation).
At present, various types of CD players are commercially available, but the regeneration signal characteristics are different from player to player, and therefore, the error rate is also different from each other. Therefore, it is necessary that the error rate is sufficiently small even when regeneration is effected by many types of commercially available CD players as far as possible.
However, the present inventors have found the following. It is said that the above-mentioned conventional optical recording mediums capable of recording can be regenerated by commercially available CD players. However, the jitter value of the pits (hereinafter called "pit jitter value") and the jitter value of the part between consecutive-pits (hereinafter called "land jitter value") is not always small. Some CD players fail to reduce error rates sufficiently. When audio information and the like are recorded, there is not so noticeable problem, but when code data and the like are recorded there are serious problems.
Further, such optical recording medium is required to be durable, but durability of conventional mediums is not always sufficient. For example, according to the present inventors' investigation, when conventional optical recording mediums are allowed to stand at 85.degree. C. at 95% RH for a long time, the characteristics of the unrecorded portions hardly change whereas the error rate at recorded portions is deteriorated to a great extent, and according to observation with a microscope of the region where error rate has been deteriorated, pits having defects of several microns with the pit at the center are found.
The present inventors have researched variously so as to eliminate drawbacks of the above-mentioned conventional recordable optical mediums having playability by CD players, and investigated the shape of groove, surface shape of recording layer containing a dye, thickness of recording layer at a groove portion and that at a land portion.
As a result, the present inventors have found particular conditions under which a sufficient radial contrast signal can be assured, there is playability by various commercially available CD players, jitter value and error rate are small and further, characteristics such as error rate and the like do not change when the CD is stored under condition of high temperature and high humidity for a long time.